My advice is to investigate one other possible lead, which is DPC latency. Basically it's a case of misbehaving driver or some other low level process, in the sense that it gets exclusive access to the CPU(s), starving your DAW just enough to not be able to process audio in real-time. Try one of these:
http://www.thesycon.de/deu/latency_check.shtml
http://www.resplendence.com/latencymon
They should indicate "green".